1. Field of the Invention
This present invention relates in general to nuclear reactor main coolant pumps and in particular to such coolant pumps that have vertically oriented shafts with lower guide bearings. More particularly, the invention relates to such main reactor coolant pumps that operate at 1,500 rpm.
2. Description of Related Art
The reactor main coolant pump motors drive the reactor main coolant pumps which are part of the primary reactor coolant system in a pressurized water reactor nuclear power plant. The motors are typically arranged with vertical shafts. The bearing and lubrication system of a vertical motor is usually contained in two separate oil pots (also referred to as oil reservoirs). An upper oil pot contains the upper guide or radial bearings and the total thrust bearing system. The lower oil pot contains the lower guide or radial bearings. Each of these oil pots is typically provided with cooling coils for carrying cooling water to dissipate the heat which is generated by the bearing systems.
The design of the oil pots is such that the oil level within the pot should be monitored during operation to ensure that the oil level is not rising above or falling below the expected levels. A rising level might indicate, for example, a water leak within the cooling coils which results in water entering the pot and mixing with the oil. If such a situation were to persist, the lubricating ability of the oil would be sharply diminished and, more importantly, the oil/water mixture would overflow the pot and migrate toward the hot reactor coolant pump, where a fire could result.
Reactor coolant pump motors that normally operate at 1,500 rpm, are installed at many facilities outside the United States wherever grid electrical power is supplied at 50 Hz. In comparison, the corresponding reactor main coolant pumps installed in the United States where the grid electrical power is supplied at 60 Hz, operate at 1,200 rpm. One type of 1,500 rpm vertically oriented reactor coolant pump motor has a baffle and support arrangement between the lower guide bearings and the lower portion of the oil pot that contains the cooling coils, to reduce turbulence of the oil that was an anticipated result from the increased speed of the coolant pumps and to avoid oil splashing over the edges of the pot and possibly migrate toward the hot reactor coolant pump where a fire could result. The lower guide bearings in these 1,500 rpm motors historically have exhibited higher than expected operating temperatures at the bearing pads. The overheating cannot be explained by the difference in speed alone. While these higher temperatures have not been known to affect operability, they typically are greater than the specified normal operating temperature of 160° F. (71.21° C.) and may run close to the normal alarm point of 190° F. (87.8° C.) when their oil cooler component cooling water approaches the maximum normal operating temperature of 105° F. (40.6° C.). These temperatures are typically achieved with component cooling water flow rates of 6 GPM nominal. The high operating temperatures may have adverse consequences for long-term bearing, shaft and oil performance, particularly with sustained component cooling water temperatures at or near maximum. Based on the principles of fluid mechanics and heat transfer and experience with over 150 60Hz reactor coolant pump motors, the inventors have concluded that these high lower guide bearing temperatures are due to the lack of oil flow within the reservoir caused by three components that do not exist in the 1,200 rpm motors. These three components isolate the portion of the oil reservoir that contains the bearings from the portion that contains the oil cooler, except for some relatively small passages in and between these components and the remaining parts of the lower guide bearing assembly.
Accordingly, it is an object of this invention to improve the oil circulation within the lower guide bearing oil pots to reduce the operating temperatures of the lower guide bearings.
It is a further object of this invention to improve the circulation of the oil within the lower guide bearing oil pot without increasing the turbulence of the oil to the point where the oil splashes over the edges of the pot.
It is an additional object of this invention to achieve the foregoing objectives without reducing the support for the bearing.